Do not pass use_prebuilt_mojo_shell to gn.

scripts/slave/recipes/chromium_mojo.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from infra.libs.infra_types import freeze
 
 DEPS = [
   'bot_update',
   'chromium',
   'path',
   'python',
 ]
 
 
 BUILDERS = freeze({
   'chromium.mojo': {
     'builders': {
       'Chromium Mojo Linux': {
-        'chromium_apply_config': ['gn_use_prebuilt_mojo_shell'],
         'chromium_config_kwargs': {
           'BUILD_CONFIG': 'Release',
           'TARGET_PLATFORM': 'linux',
         },
       },
     },
   },
 })
 
 def _RunApptests(api):
   apptest_runner = api.path['checkout'].join('mojo', 'tools',
       'apptest_runner.py')
   apptest_config = api.path['checkout'].join('mojo', 'tools',
       'data', 'apptests')
-  api.python('run apptests', apptest_runner,
+  api.python('app_tests', apptest_runner,
              [apptest_config, api.chromium.output_dir])
 
 
 def GenSteps(api):
   _, bot_config = api.chromium.configure_bot(BUILDERS, ['gn'])
 
   api.bot_update.ensure_checkout(force=True,
                                  patch_root=bot_config.get('root_override'))
 
   api.chromium.runhooks()
 
   api.chromium.run_gn(use_goma=True)
 
   api.chromium.compile(targets=['html_viewer_unittests',
                                 'mojo/services/network',
                                 'mojo/services/network:apptests'])
 
   api.chromium.runtest('html_viewer_unittests')
 
   _RunApptests(api)
 
 
 def GenTests(api):
   for test in api.chromium.gen_tests_for_builders(BUILDERS):
     yield test